1. Field of the Invention
The present invention concerns a polymer dispersion type liquid crystal element capable of controlling reflectance and transmittance by the application of an electric field or a magnetic field, as well as a manufacturing method thereof. The polymer dispersion type liquid crystal element manufactured by the present invention is applicable as optical elements such as displays, light control elements and light modulation elements.
2. Related Art Statement
As a display element and a light control element, a polymer dispersion type liquid crystal (PDLC) in which liquid crystals are dispersed in interstices of polymers of a three-dimensional structure as shown in FIG. 1 has been studied. In PDLC, incident light is refracted at an interface due to the difference between a refractive index of the liquid crystal in the interstice and a refractive index of the polymer and passes through a number of droplets in the entire film to provide a scattered state in a state not applied with voltage (FIG. 1 (a)) . On the other hand, when the voltage is applied, the liquid crystals are oriented perpendicular to a substrate in which the refractive index in the direction of the major axis is aligned with the refractive index of the polymer to provide a transparent state (FIG. 1(b)). The technique of PDLC requires no polarization plate, for which application to a projector light bulb is studied and bright display is expected.
The structure of the three-dimensional polymers includes those having interstices in which liquid crystals are distributed are present independent of each other or distributed continuously. As the method of manufacturing such polymer dispersion type liquid crystals, the following methods, generally classified into three types, are proposed.
At first, a method of impregnating liquid crystals into a porous polymer in which liquid crystals can be oriented at random. Secondly, a method of mixing and emulsifying a polymer and liquid crystals in a solvent and then evaporating the solvent to vulcanize the polymer. Thirdly, a method of mixing a monomer, an oligomer or a mixture thereof with liquid crystals to form a polymerizable composition, and polymerizing the composition, for example, by irradiation of heat or ultraviolet rays, by which the polymerized polymer and liquid crystals are put to phase separation.
SPIE.1080, 83, (1989) discloses a polymer dispersion type liquid crystal element in which the refractive index changes periodically inside as application of the polymer dispersion type liquid crystals. Specifically, as shown in FIG. 2, a structure in which polymer layers and polymer dispersion type liquid crystal layers are laminated alternately is manufactured to attain a layered structure in which the refractive index changes periodically. In this case, in a state where voltage is not applied, reflection light is formed based on the principle of an interference filter due to the periodical refractive index difference between the polymer dispersion type liquid crystal layer and the polymer layer (FIG. 2(a)). Further, when the voltage is applied, the refractive index of the polymer dispersion type liquid crystal layer is aligned with that of the polymer layer to provide a transparent state (FIG. 2(b)).
As shown in FIG. 2, in the existent polymer dispersion type liquid crystal element in which the refractive index changes periodically in the inside, orientation of low molecular (low molecular weight) liquid crystals in the droplets of the polymer dispersion type liquid crystal layer is at random for the entire polymer dispersion type liquid crystal layers. Accordingly, the refractive index of the polymer dispersion type liquid crystal layer is lowered to a value {n.sub.o (3-v)+n.sub.e }/3, in view of the primarily approximated refractive index (n.sub.e +2n.sub.o)/3 of the droplet, a value of the refractive index (n.sub.p : about n.sub.o) of the polymer in the polymer dispersion type liquid crystal layer and a value of the volumetric fraction (v) of the low molecular liquid crystal to the polymer in the polymer dispersion type liquid crystal layer. In this case, n.sub.p is a refractive index of the polymer, n.sub.o is a refractive index where the vibration direction of an electric field of light is perpendicular to the major axis of the liquid crystal molecule, and n.sub.e is a refractive index where the vibration direction of the electric field of light is in parallel with the major axis of the liquid crystal molecule. Therefore, the difference of the refractive index between the polymer dispersion type liquid crystal layer and the polymer layer is reduced to result in a problem that no high reflectance can be obtained.
Accordingly, in order to improve the reflectance of the polymer dispersion type liquid crystal element in which the refractive index changes periodically inside, a technique for aligning the orientation of the low molecular liquid crystals in the element has been sought, but such a technique has not been found yet.
As analogous techniques, several techniques for controlling the orientation direction of low molecular liquid crystals in droplets at the initial stage in PDLC have been proposed.
For example, (1) U.S. Pat. No. 5,188,760 discloses an orientation controlling technique by the combination of PDLC using a liquid crystal polymer and an orientation film. In this technique, a liquid crystal monomer is used for a polymerizable composition as a precursor of PDLC and the composition is injected into a cell having an orientation film. UV-light or heat is applied in this state to conduct polymer phase separation in a state where the liquid crystal polymer as a polymer of the liquid crystal monomer and low molecular liquid crystals are oriented in the direction of the orientation film, and orientation of the low molecular liquid crystals is fixed after vulcanizing of the liquid crystal monomer.
Further, (2) Japanese Published Unexamined Patent Application No. Hei 5-281527 discloses an orientation controlling technique by the combination of PDLC with a horizontal external magnetic field or electric field. In this technique, a polymerizable composition is injected into a cell with no orientation film and UV-light or heat is applied to the cell in a state of applying an external magnetic field or electric field in a horizontal direction, thereby conducting polymer phase separation in a state where the low molecular liquid crystals are oriented in the direction of the external magnetic field or electric field and fixing the orientation of the low molecular liquid crystals after vulcanizing the polymerizable composition.
Further, (3) Japan Display '92, 699 discloses an orientation controlling technique by the combination of PDLC and an orientation film. In this technique, a polymerizable composition as a precursor of PDLC is prepared so as to form a liquid crystal phase at an extremely high liquid crystal concentration, which is injected into a cell with an orientation film. In this state, the polymerizable composition in the form of the liquid crystal phase is oriented along the direction of the orientation film and, by conducting polymer phase separation inside the cell with the orientation film under the application of UV-light or heat in this state, orientation of the low molecular liquid crystals is fixed while keeping the state of the initial orientation.
Furthermore, (4) Mol. Mat., 2,295 (1993) discloses an orientation controlling technique using a polymer compound having a photo-dimerizable structure. In this technique, manufacture is conducted by an impregnation method using a polymeric compound having a photo-dimerizable structure as a material. At first, a composite film comprising a polymeric compound having a photo-dimerizable structure and a poor solvent to the polymeric compound is prepared and the poor solvent is extracted from the film, followed by drying to prepare a porous polymer comprising the polymeric compound having the photo-dimerizable structure. Further, the porous polymer is impregnated with low molecular liquid crystals to prepare PDLC comprising the polymeric compound having the photo-dimerizable structure. PDLC is irradiated with polarized light to cause photo-dimerization. The low molecular liquid crystals are oriented along with the structural change of the polymeric compound by the photo-dimerization.
Among them, however, the composite structure as shown in FIG. 2 could not be prepared by the orientation and fixing methods of (1) and (3) since the orientation film had to be used. Further, in the orientation fixing method (2), the external magnetic field or an electric field had to be applied in parallel with the cell but it was extremely difficult to apply an effective external magnetic field or electric field throughout the plane of the cell if the cell size was large.
For instance, Japanese Published Unexamined Patent Application No. Hei 5-281527 describes that the level of the external electric field has to be 1 kV/cm or higher in a case of applying the external magnetic field. Assuming the diagonal cell size to be 12 inches, it is calculated that about 350 kV or higher of an application voltage is necessary but the application voltage of such a high level cannot be attained easily. Further, in the method (4), since the porous polymer is prepared by use of the polymeric compound having the photo-dimerizable structure, injection of the liquid crystals has to be conducted by the impregnation method. Further, it is necessary for the film obtained in (4) that all interstices have to be in communication with the film surface for the impregnation of the liquid crystals. Independent interstices are not desirable since they cause residue of water or bubbles thus giving undesired effect on the reliability of the characteristics. Further, since the film has a flexibility, the film is liable to be deformed in the step of removing water and impregnating the liquid crystals, and it is difficult to control the film thickness upon appending opposing substrates to each other. Furthermore, in the method (4) , it is impossible to manufacture a composite structure as the periodical structure shown in FIG. 2.
As described above, any of the existent liquid crystal orientation methods involves problems, and a polymer dispersion type liquid crystal element having a sufficient reflectance and in which the refractive index changes periodically inside has not been obtained yet.